A field-effect transistor (FET, also referred to as a MOSFET) relies on an electric field to control the potential of the free-carrier concentration in, and hence the conductivity of a “channel” in a semiconductor material. The voltage applied between the gate and source terminals modulates the current between the source and drain of the FET.
MOSFETs are made with semiconductor processing techniques, common in the field of semiconductor manufacturing. The most common use of MOSFETs is the CMOS (complementary metal oxide semiconductor) integrated circuit which is the basis for most digital electronic devices. FETs are also used in analog devices. In digital integrated circuit technology, an output signal in response to the input signal is either “on” or “off”, which is designed to assure as rapid as possible a switch of an output signal from the “on” to “off” state (and vice versa) with as little energy as possible and with maximum noise rejection. In analog integrated circuit technology an output signal follows continuously an input signal.
FETs for digital applications have requirements which differ from those for analog applications. For example, it is very important to maintain minimum dimensions of the channel, and hence gate-length, of a digital application FET; this results in high drain current and in low gate capacitance, both of which contribute to fast switching speed. Furthermore, high halo concentrations are required in such short-channel FETs to maintain effective control of the channel by the gate. Thus it is important to maintain good linewidth control of FET gates in digital applications. By contrast, analog applications typically demand more-ideal output characteristics such as low drain conductance (Gds) and good threshold voltage tolerance and matching. These characteristics are attained with longer channels and thus longer gate length FETs, and typically with weaker halo doses. Thus the structural and electrical demands differ between the two applications, digital and analog.
In order to fabricate an integrated circuit with FETs for both analog and digital applications, it is important to thus maintain different doping and electrical characteristics of these devices. In doing so, during the extension and halo implant of the FETs for the digital application, it is important to protect the FETs for the analog devices, and vice versa. This will ensure that the dopants intended for the digital FETs will not reach under underneath the channel region of the analog FET gates, which are not being doped in these steps. In order to provide analog FET and digital FET doping profiles that are independent of one another, one must selectively cover (typically with photo-resist) one set of devices, e.g. the analog FETs, while performing extension and halo ion implantations for the second set, e.g. the digital FETs, and then, remove the covering resist, cover the second set of devices, e.g. the digital FETs, selectively, and perform the extension and halo ion implantations for the second set of devices. This is an expensive and time-consuming process.